Porous gas diffusion elements have been used since the 1920's for bubbling air into sewage in the activated sludge process.
Such elements are formed of a body of solid particles which has been shaped, pressed and rendered coherent by bonding or sintering in a compacted form having pores. Such compacts have been manufactured in a variety of forms of plate or disc-like configuration and mounted in holders in or near the bottom of sewage treatment tanks. Air under pressure from a plenum beneath the element, is forced upwardly through pores extending through the body of the element to its upper surface, from which the air is released in the form of bubbles whose fineness is controlled in part by the sizes of the pores at the upper surface. The air encounters some resistance in passing from beneath the element into the water, and it is widely known that this includes frictional losses resulting from passing the air through the fine pores of the element.
With the exception of occasional defective elements which are inevitably produced in most manufacturing processes, the manufacturers of diffusion elements have apparently assumed that the quantity of air released from their upper surfaces was distributed with reasonable uniformity across the entire surface of the element. Although highly detailed design and performance specifications are regularly applied to most components of sewage aeration systems, stringent distribution uniformity specifications have not been developed for diffusion elements. Also, a widely accepted test for uniformity of air distribution in a sewage aeration air diffusion element has been to merely make a visual examination of the bubble pattern emitted by the element while it is operating submerged in water. Moreover, persons skilled in the art have accepted this type of test for many years. They have done so despite the fact that it is quite difficult to visually ascertain whether a submerged diffusion element is distributing air uniformly, due to the disturbance created by discharge of bubbles into the water. Moreover, if accurate methods have existed heretofore by which one could compare the air output of different portions of the area of a diffusion element, such techniques have not been generally known and applied in commercial practice by persons active in the manufacture of sewage aeration diffusion elements and associated aeration systems. From this it might appear that there is little or no need for detailed or stringent specifications for the air flow distribution uniformity of diffusion elements for aeration.
A bubble release pressure test developed by the present applicants has made it possible to compare the relative ease with which different portions of the gas discharge surface of a diffusion element will discharge bubbles. Through the use of this test it has been found that the gas distribution properties of diffusion elements are not nearly as uniform as previously supposed. Although randomly disposed disuniformities of gas distribution have been observed, use of the bubble release pressure test referred to above has shown a trend for some diffusion elements to discharge a disproportionate amount of their total flow through a central zone. A larger quantity of flow through a given zone results in an increased rate, which tends to produce larger bubbles. Due to their reduced surface area per unit volume, larger bubbles tend toward reduced gas transfer efficiency, e.g. OTE, oxygen transfer efficiency. Thus, in a sewage aeration process, passing a disproportionate share of the total flow through the central portion of the diffusion element, while the outward or surrounding zone of the element is underused, produces excessively large bubbles and therefore reduced oxygen transfer efficiency.
The tendency to release a disproportionate share of the total flow through a central zone may arise from a variety of causes. For instance, a diffusion element whose permeability is substantially uniform across its gas release surface may release an excessive proportion of gas in its central portion due to the design of associated components, such as, for example, the holder or mount for the diffusion element. The configuration of the holder or mount may concentrate flow through the center of the element. Also, diffusion elements are known which have been manufactured in such a manner as to provide lesser permeability, greater density or lesser height in a peripheral annular zone of relatively small proportions. For example, U.S. Pat. No. 4,046,845 to Richard K. Veeder discloses the concept of subjecting a relatively narrow annular zone of a diffusion element to sufficient extra pressing to prevent discharge of bubbles from said zone. Application of the above described bubble release pressure test to such elements has shown that the effects of the extra pressing extend a considerable distance into the element from the annular zone, thereby considerably affecting the air distribution through the element and providing substantial encouragement for disproportionate flow through the central zone of the element. The present invention is aimed at the correction of these difficulties.